The present invention relates to the combustion of oil shale in a circulating fluidized bed (CFB) boiler.
In the furnace of a CFB boiler, carbonaceous fuel, such as coal or biofuel, is combusted in a bed of inert material, such as sand, and fluidized oxygenous gas, usually air. The upward velocity of the fluidizing gas in the furnace is usually 5-10 m/s, so as to perform the combustion in a vigorously turbulent bed of particles entrained with the fluidizing gas. Most of the particles escaping from the furnace of a CFB boiler with the flue gas produced in the furnace are separated from the flue gas, usually in a cyclone separator, and are returned to the lower portion of the furnace.
Oil shale, found, for example, in Estonia, the Middle East and North Africa is a special kind of carbonaceous fuel. It comprises 25-40% fossil organic material, in dry mass, with the rest being mineral material having calcium carbonate as the main component. The organic material comprises 85-90% of combustible, volatile matter, and typically, about 1.8% of sulfur and 0.75% of chlorine. Due to the chlorine, combustion of oil shale suffers from the generation of high corrosion. Another problem related to oil shale is that it is very friable, producing a high amount of fly ash, which tends to foul the heat transfer surfaces in the flue gas path.
Usually, in CFB boilers, only a portion of the combusting air is introduced as primary air through the bottom grid of the furnace. The rest of the oxygen needed for the combustion is introduced as secondary air at higher levels in the furnace, usually 2-6 m above the bottom grid.
The split between primary air and secondary air depends on the type of fuel. When combusting typical fossil fuels, such as bituminous coal, the proportion of primary air is usually from about 55% to about 65%. With lignite and biofuels, the proportion of primary air is usually about 55%, or as low as 40%, if limestone is introduced to the furnace for reducing sulfur oxide emissions.
According to a commonly used design, the bottom section of the furnace of a CFB boiler is downwards tapering so as to maintain an approximately uniform fluidizing velocity at all levels of the boiler, despite the fact that a part of the combustion air is introduced as a secondary air. Correspondingly, the grid area of the furnace varies typically between 40% and 55% of the cross-sectional area of the furnace at higher levels, when the proportion of primary air varies between 40 and 65% of the total combustion air.